The present invention relates to a measuring method of an optical heterodyne interference and a measuring apparatus thereof, and in more details, it relates to a measuring method of an optical heterodyne interference and a measuring apparatus thereof, for enabling an improvement on measuring accuracy while reducing noises on an interference light, in the heterodyne interference measuring apparatus for measuring a recess concave defect, a concave-convex defect or a relative shift amount on a target (i.e., a work), such as, a testing apparatus for detecting a surface defect on a magnetic disk or a substrate thereof, etc., or a surface testing apparatus of the wafer, etc., or a flatness measurement of the wafer, etc.
In accordance with the optical heterodyne interference measuring method, while irradiating a reference light and a measuring light, which are generated lightly differing from each other (from several kHz to several hundreds kHz), upon a target to be measured, so as to bring about interference between the reflection light upon the target and the reference light, and further applying an AC offset frequency (i.e., a beat frequency) onto a DC-like interference fringe obtained through the conventional method, measurement is made on the shift of the target or the like, from a phase change on this AC signal.
This measuring method, since it can be hardly affected from ill influences due to variation of a light amount of a light source, is suitable for detection (measurement) of the condition on the target of low reflectivity, in particular, the condition of a projection and/or concave defect(s) on the magnetic disk and the substrate thereof.
Testing on the projection defect on the surface of the magnetic disk and the substrate thereof with using the optical heterodyne interference measuring apparatus is already known, for example, in Japanese Patent Laying-Open No. 2003-207457 (2003), which was filed by the same applicant.
Also, a capacitance sensor is used for the purpose of measuring a relative shift amount of the target, however in case when measuring it with such the capacitance sensor or the like, in normal, since the measurement is made on the distance from a reference position to a surface of the work, then errors are generated in the reference position because of expansion or contraction due to changes of temperature and secular change of the frame for supporting a member of that reference position, and therefore due to this, there is a problem that it is difficult to make the measurement thereof with high accuracy. Then, various kinds of measurements are conducted, including a flatness measurement with utilizing the optical heterodyne interference measuring apparatus, etc.